Steam cracking furnaces have long been used to crack a variety of hydrocarbon feedstocks to ethylene and other valuable olefinic gases. For the past 20 or 30 years cracking at short residence time and high temperature has been favored for its beneficial effect on selectivity to ethylene. Basic designs of such short residence time-high temperature steam cracking furnaces are illustrated by U.S. Pat. No. 2,671,198 (dated Jun. 20, 1972) and U.S. Pat. No. 4,342,642 (dated Aug. 3, 1982).
When thermally cracking a saturated hydrocarbon down to olefinic hydrocarbons--such as the cracking of ethane to predominantly ethylene or the cracking of heavier saturated hydrocarbons like those comprising a naphtha or gas oil feedstock down to ethylene and other higher olefins--in order to maximize the conversion and the selectivity of such cracking conversion of the saturated hydrocarbon feedstock into ethylene, it is desirable to input that quantity of heat (Q) needed to effect cracking of the saturated hydrocarbon feed very rapidly while minimizing the time that the initial cracking product--namely, ethylene--is exposed to this quantity of cracking heat. To fast crack the saturated hydrocarbon feed to ethylene and then quickly remove this so formed ethylene from this high heat environment maximizes the final yield of ethylene for the degree of conversion obtained. This then is the concept that underlies the millisecond residence time at a high temperature which is now the preferred mode for furnace cracking of saturated hydrocarbon feeds to olefin products.
A steam cracking furnace comprises a refractory lined firebox containing a multiplicity of high alloy metal cracking lines through the interior passage of which flows the hydrocarbon feedstock to be cracked, together with a suitable amount of diluting steam. The sensible heat and the heat of cracking are supplied by burners located on the floor and/or walls of the firebox and this heat transfers through the metallic materials of these reaction lines into the hydrocarbon feedstock that flows there-within. A metallic cracking line can be as long as 400 feet and coiled in a serpentine shape that runs vertically up and down in the firebox, or it may be as short as 40 feet in a straight single pass through the firebox, such as the design described in the U.S. Pat. No. 4,342,642 cited above.
Cracking furnaces, as constructed today, provide for millisecond residence time at high temperatures and are, with respect to their radiant heating cracking reaction lines, constructed of metallic materials. The fireboxes themselves, since these are lined with refractory materials, are capable of delivering a greater heat load than the metallic materials of the radiant cracking reaction lines located within the firebox can withstand. This maximum service temperature of the metallic materials of which the cracking reaction lines are constructed then dictates a long line in order to accomplish the desired quantity of heat (Q) input into the hydrocarbon mass flow therethrough for that short (milliseconds) time of residence of this hydrocarbon mass within the metallic cracking reaction line. Either this, or the time of residence of the hydrocarbon mass, including its ethylene content, within the metallic reaction cracking line must be increased.
Given the extreme conditions to which the materials of the cracking reaction lines are exposed in a thermal cracking operation--which involve thermal expansion and contractions of such materials as they are suspended within the firebox which radiantly heats them--to date, metallic materials have been regarded as the only materials practical for construction of such cracking lines. The strength and serviceability dictated by the dimensions required by a cracking line in order to achieve the needed rapid transfer of heat to accomplish the level and degree of cracking desired within the short residence times that are desired have, here to date, dictated the use of metallic materials for their construction.